Obstructive sleep apnea (OSA) is the most common sleep disorder and is responsible for more mortality and morbidity than any other sleep disorder. OSA is characterized by recurrent failures to breathe adequately during sleep (termed apneas or hypopneas) as a result of obstructions in the upper airway.
Apnea is defined as a complete cessation of airflow. Hypopnea is defined as a reduction in airflow disproportionate to the amount of respiratory effort expended and insufficient to meet the individual's metabolic needs. During an apnea or hypopnea, commonly referred to as an abnormal respiratory event, oxygen levels in the brain decrease, while the carbon dioxide levels rise, causing the sleeper to awaken. During an apneic event, adrenaline and cortisol are released into blood and the heart rate and blood pressure increase. The brief arousals to breathe are followed by a return to sleep.
OSA is a serious yet treatable health problem worldwide. Published reports indicate that untreated OSA patients are three to five times more likely to be involved in industrial and motor vehicle accidents and have impaired vigilance and memory. Untreated OSA leads to hypertension, stroke, heart failure, irregular heartbeat, heart attack, diabetes and depression. Current estimates reveal that over 80% of individuals with moderate to severe OSA remain undiagnosed.
The current standard for the diagnosis of OSA is an expensive overnight sleep study—polysomnography (PSG), which is administered and analyzed by a trained technician and is reviewed by physician specializing in sleep disorders. A typical overnight PSG includes recording of the following signals: electroencephalogram, electromyogram, electrooculogram, respiratory airflow (with oronasal flow monitors), respiratory effort, oxygen saturation (oximetry), electrocardiography, snoring sounds, and body position. These signals offer a relatively complete collection of parameters from which respiratory events may be identified and OSA may be reliably diagnosed.
Obstructive apnea and hypopnea are defined as absence and reduction, respectively, in airflow, in spite of continued effort to breathe, due to obstruction in the upper airway. Typical polysomnography includes some recording of respiratory effort. The most accurate measure of effort is a change in pleural pressure as reflected by an esophageal pressure monitor. Since the esophageal pressure monitoring method is difficult to administer and highly uncomfortable to patients, other methods have been developed. These methods estimate respiratory effort and depend on measures of rib cage and abdominal motion and include inductance or impedance plethysmography, or simple strain gages.
The expense, inconvenience and complexity of traditional PSG sleep studies have created a significant need for simplified and cheaper OSA diagnostics. As a result, several portable sleep monitors have been developed over the past several years. These monitors measure fewer parameters than PSG, yet some of them offer the accuracy comparable to that of PSG. Furthermore, portable OSA monitors bring the convenience of in-home testing and are often shipped to patients after being prescribed by physician. While significantly less complex than sleep lab PSG equipment, most in-home OSA diagnostic systems require the patient to apply sensors, plug in wires, apply and adjust transducers, straps, gages and other measuring devices, or to operate a computer-controlled bedside unit. This equipment can be difficult for a lay person to apply and properly operate. One of the existing home diagnostic systems Watch-PAT, manufactured by Itamar Medical Ltd., requires patients to watch an hour-long training presentation prior to using its product. Another example of a difficult to use diagnostic system is AccuSom manufactured by NovaSom. The difficulty that patients experience in setting up in-home devices limits compliance, results in poor quality of sleep data and limits the adoption of the in-home sleep monitors. Furthermore, the equipment can be uncomfortable and the quality of the sensed data can be poor due to motion artifacts or sensors getting displaced during sleep. One of the main reasons for poor quality of sleep data is the currently available respiratory effort sensors. Respiratory effort sensors are typically designed as chest or abdominal bands measuring chest expansion and are based on inductive plethysmography, piezo-electric crystals, conductive elastomeres, magnetometers and strain gauges. These respiratory effort sensors are particularly prone to motion artifacts and trapping. Occurrence of trapping artifacts, as a patient turns from one side to another, may significantly affect the quality of respiratory effort data. Two studies found the failure rate for effort bands ranged between 7% and 21% even when the bands are applied by a trained sleep study technician.
Thus, a device that eliminates or reduces the use of wires, and can be reliably self-applied with minimal instruction would be beneficial to accurately diagnose patients at risk for OSA. Furthermore, a device that can detect respiratory effort without the use of an effort band would offer the convenience and improve the quality of sensed respiratory effort data by eliminating trapping artifacts.
There is a known device—ARES—manufactured by Watermark Medical Inc., which detects respiratory effort by assessing “forehead venous pressure” and is based on an algorithm combining signals from a photoplethysmography sensor, a pressure sensor and an accelerometer. The device includes nasal tubes for the assessment of nasal airflow and a bulky main unit that is attached to the patient's forehead with straps. Due to its form factor and size, the use of airflow tubes and the types of sensors used, the device is uncomfortable to sleep in, and may be prone to sensor displacement and poor quality of sleep data.
There is also a known device—SleepStrip, which incorporates three thermistors to measure oronasal airflow, a battery, a microcontroller and a memory in a strip which is applied with adhesive to patient's face. However, due to the lack of the sensors for oxygen saturation or respiratory effort, this device is only suitable for screening patients for abnormal airflow and is not sufficient for the detection of OSA. While abnormal airflow is a key symptom of OSA, the respiratory effort during apnea and hypopnea is the physiologic parameter that distinguishes OSA from other forms of sleep-disordered breathing such as central sleep apnea.
One method proposes the use of photoplethysmography for the detection of respiratory effort without the use of effort belts. When analyzing the photoplethysmography (PPG) signal during an apneic event, it has been suggested to use a low-pass filter or a frequency analysis to identify respiratory induced intensity variation (U.S. Pat. No. 7,690,378). However, these intensity variations are not exclusively due to respiratory effort and therefore, an application of a low-pass filter or a frequency analysis is insufficient to identify respiratory effort during apneic events. There are several possibilities as to the origin of these intensity variations. Inspiration results in a momentary reduction in stroke volume and, therefore, a corresponding reduction in cardiac output, which has an effect on the pulsatile component of the PPG waveform. Also there are blood volume changes during the respiratory cycle due to the transmitted changes in intra-thoracic pressure. Additionally, it has been shown that sympathetically mediated vasoconstriction of the arteries also plays a part in PPG intensity variations. It is desired to have a method and system that eliminates the role of sympathetically mediated vasoconstriction from PPG intensity variations during apneic events in order to accurately identify respiratory effort.
An easy-to-apply and easy-to-operate diagnostic device, which incorporates the sensors for accurate detection of respiratory events and respiratory effort, is still needed for convenient in-home sleep apnea diagnosis.